Scaling of midgut perimeter and amino acid transporter expression in Manduca sexta larvae

Abstract

Metabolic rate of tobacco hawkmoth (Manduca sexta) larvae increases with body mass to the approximately 0.90 power, greater than the 0.67 and 0.75 scaling exponents predicted by isometric increase of surface area and fractal geometry of exchange networks. We evaluated whether increased midgut folding and/or nutrient transporter expression in larger larvae contribute to this discrepancy. In 3rd through 5th instar larvae, midgut wet mass scaled with an estimated exponent of 0.85. Exponents for anterior, middle, and posterior midgut were 0.96, 0.70 and 0.85, respectively. Composite micrographs of transverse sections revealed progressively increased folding of the midgut perimeter in 1st through 5th instar larvae. These results suggest that relative midgut surface area declines less than predicted by theoretical models as larvae grow. Also, expression of mRNA encoding a potassium amino acid transporter (KAAT1) increased 2 to 3‐fold in 5th instar compared to 4th instar larvae. To assess the consequences of altering the expression of KAAT1, we used RNA interference to decrease midgut KAAT1 expression. Delivery of a 453bp dsRNA encoding KAAT1 through direct injection into the hemolymph resulted in an approximately 6‐fold reduction in KAAT1 mRNA expression, while oral delivery in a sucrose droplet did not reduce KAAT1 expression. Knockdown of KAAT1 mRNA expression did not affect larval growth. (NSF‐DMS0827208)